1. Technical Field
The invention is related to methods of forming ceramic matrix composites using fibers coated with a toughening layer that protects the fibers from advancing cracks in the ceramic matrix.
2. Background Art
Fiber reinforced ceramic matrix composites comprise a framework of ceramic fibers embedded in a ceramic matrix. The mechanical properties (e.g., toughness, strength and strain to failure) of such composites can be significantly improved by providing a ductile coating over the fibers that is stable and preferably resistant to oxidation and that defeats advancing cracks from propagating through the fibers in the composite. Fracture of the brittle fibers is defeated by the ductile coating because the ductile material blunts advancing cracks and absorbs strain energy. The resistance to fiber failure can be further improved by the ductile coating because it accommodates excessive strains caused by any mismatch in thermal expansion between the fiber and matrix, and because the relatively weak shear strength and the ductility of the coating allow the fiber to bridge the crack opening without prematurely failing the brittle fibers. Even further improvement can be achieved by providing an optimum interfacial shear strength between fiber and ductile coating, and/or between the ductile coating and matrix, such that the fibers pull out in the wake of the cracks. This pull-out will dissipate additional strain energy due to frictional sliding of the fiber through the matrix. These features enhance the toughness and strength of the composite because the unbroken fibers hold the composite together and bear the load in the wake of advancing cracks in the matrix. The history of development of this technique is described in U.S. Pat. No. 4,885,199 to Corbin et al.
For certain applications, the ceramic matrix composite must be stable at temperatures above 2200 degrees F. in an oxidizing environment. Well-known fiber coatings such as carbon and boron nitride are not stable under such conditions. Any material to be substituted for a fiber coating in place of the carbon or boron nitride must be both resistant to oxidation and must possess sufficient strength to transfer loads from the matrix to the fiber while having the necessary properties to protect the fibers from failure caused by advancing cracks in the matrix and to extend the bridging characteristics of the fibers in the wake of the crack. Furthermore, the coating must be easy to apply to yarns and to weaves of ceramic fibers during manufacturing. This approach is also useful in certain applications where the temperatures are intermediate, for example, 800 to 2000 degrees F. Moreover, in some applications, where the environment will not be oxidizing, the ductile metal could be any one of many candidates including refractory metals and alloys and transition metals and alloys, such as Ni, NiCr or FeCrAlY.
U.S. Pat. No. 3,869,335 to Siefert discloses metal-coated glass fibers in a glass matrix, where the metal layer acts as a diffusion barrier to protect the fibers from reacting with the glass matrix. Since there is a glass coating over the metal coating, Siefert is not directed to high or intermediate temperature ceramic composites. U.S. Pat. No. 3,943,090 to Enever discloses carbon fibers in synthetic plastic with an elastomer coating on the carbon fibers, and is not directed to high temperature ceramics with a ductile metal coating for the fibers.
The prior art as described in U.S. Pat. No. 4,885,199 referenced above typically relied upon the characteristics of the chemical bond between the fiber coating and the fiber or intrinsic properties of the coating such as cleavage to achieve desired characteristics, such as toughening. For applications in the high temperature oxidizing environments described above, the chemical bond between the coating and the fiber would have to provide all of the necessary features, including crack blunting, crack deflection, fiber de-bonding and pull-out as well as resistance to oxidation and high temperatures. The problem with this approach is that it is very difficult to select the best fiber coating material for a given ceramic fiber so as to optimize all of the foregoing features in the same chemical bond or to select a coating with intrinsic laminar shear failure at the right strength.
In the present invention, the desired mechanical features for promoting fiber toughness are realized by depositing a ductile or soft metal coating on the fibers prior to forming the ceramic matrix. Preferably, the coating is a ductile metal which is resistant to oxidation and is stable at intermediate to high temperatures. The ductile characteristic of the metal is the mechanical property that provides the features necessary for toughness. The chemical properties of the metal coating are selected to provide the requisite imperviousness to oxidation.
As one example, the ductile metal coating in one embodiment is a metal selected from the group of noble metals, such as platinum or iridium, formed in a coating or film covering the fibers in a thickness on the order of between 0.1 and 1 micron. Platinum is the most oxidation resistant of all metals, but it does react with silicon, requiring a diffusion barrier when coating ceramic fibers containing silicon. The coefficients of thermal expansion of platinum and iridium are high compared with silicon based ceramic fibers, but the thermal expansion will be accommodated by the high ductility and relatively low elastic modulus of platinum or iridium. The thermal expansion of platinum or iridium is more compatible with that of Al.sub.2 O.sub.3 -based ceramic fibers.
Coating a ceramic fiber which is to be immersed in a ceramic matrix with a coating consisting of a noble metal is disclosed in U.S. Pat. No. 4,933,309 and U.S. Pat. No. 4,921,822 both to Luthra. Luthra discloses that the noble metal is deposited by metal sputtering, chemical vapor deposition, electroplating, or electroless plating or any combination of these processes. The problem is that such processes are relatively difficult to perform rapidly on a large production scale while maintaining a uniform coating of the metal around all of the fibers in the fiber weave. Therefore, there is a need to provide a method for coating a ceramic fiber or a weave of such fibers with a noble metal film prior to incorporating the fiber or weave into a ceramic matrix which enables the coating process to be performed more reliably and uniformly and with greater ease and rapidity on a large production scale than has been possible heretofore.
A major problem that is not addressed by Luthra is how to form a noble metal coating on non-oxide ceramic fibers, such as silicon, carbide and silicon nitride. Luthra was concerned only with oxide-based ceramics, such as alumina, which is much more resistant to oxidation at high temperatures than the non-oxide ceramics. Accordingly, Luthra was free to use any coating process, including those performed at intermediate temperatures of around 2000 degrees F. (such as chemical vapor deposition). Therefore, the art does not address the problem of how to form a noble metal coating on a non-oxide ceramic fiber in a manner which takes into account the greater tendency of the non-oxide ceramic fiber to react with the noble metal at high temperatures (compared with oxide-based ceramic fibers). The noble metal deposition processes taught by Luthra do not take into account this tendency of non-oxide ceramic fibers.
Accordingly, it is an object of the invention to provide a method for coating ceramic fibers prior to their immersion or inclusion in a ceramic matrix with a noble metal film without having to perform sputtering, chemical vapor deposition, electroplating or electroless plating and with a rapidity and reliability superior to that achieved in such processes.
It is a related object of the invention to form a highly uniform noble metal coating on a non-oxide ceramic fiber without having to elevate the temperature of the fiber or otherwise radiate energy to the fiber, in a process which thus takes into account the greater tendency of the non-oxide ceramic fiber to react with the noble metal at high temperatures (compared with oxide-based ceramic fibers).